A festive summer cocktail party in Chilmark ended abruptly in chaos, fright and injuries last night when a second story deck collapsed on a home overlooking Clam Cove in Nashaquitsa Pond.

About 50 people were attending the multi-generational party at the home of Gayle K. Lee, the hostess, when a section of the outdoor deck gave way. About 20 people were on the L-shaped deck at the time of the accident, and more than half of them went sliding or crashing some 12 feet to the ground, along with all the accouterments of a cocktail party, including glasses, dishes and clay flower pots.

The accident occurred at about 6:45 p.m.

"It was like an earthquake," said James Langlois, a Chilmark resident who attended and was on the deck at the time of the accident. Mr. Langlois was on the small part of the deck that did not collapse.

One man grabbed a table and held it, preventing it from sliding off the deck into the small crowd of distraught people who had tumbled onto the ground.

A three-year-old girl who is the granddaughter of the hostess was saved from injury when an older woman standing on the ground caught her as she slid off the collapsing deck.

"Everything fell in slow motion, with people sliding to the ground," said one guest who was among those who fell but was not injured and did not wish to be named.

"I was in the kitchen with a knife in my hand and I heard the start of the sound and saw the heads starting to get shorter," said BZ Riger-Hull, who was working for the party caterer.

"It was a horrible noise, a roiling kind of noise like the side of a mountain sloughing off or like you have a shelf with a lot of stuff sliding, and then there were people noises added to that," she said, adding: "So there was the deck with a tent on it and all these people having a lovely view and a lovely time and there was this noise - and then no people, and lots of screaming."

The scene that followed was pure chaos for about two hours. A call was placed to 911 and the initial report said that people were trapped under the deck, a report that proved to be untrue.

The emergency response was immediate and profuse. A mass casualty plan was activated, and within minutes police officers and volunteer firefighters responded to the scene from every Vineyard town. Up-Island rescue workers arrived first. Ambulances were dispatched to the scene from every town except Edgartown, where the ambulance was on standby.

At the Martha's Vineyard Hospital an emergency protocol went into effect; the emergency room staff included two surgeons, two anesthesiologists, three emergency room doctors, two internists, one orthopedist and a radiologist.

"The rescue was wonderful. All the guests got out of the way. The caterers were handing out ice and washcloths to wrap around people," said Wendy Weldon, a guest at the party. "The EMTs took care of everyone. It was absolutely incredible," she added.

One guest who was a doctor pitched in to help out with injuries.

At the time of the accident children and grandchildren were mingling with the guests, carrying plates of food. The hostess, her daughter and two grandchildren were among those transported to the hospital for treatment.

In the end the injuries were not extreme and most were not serious. Ten people were taken to the Martha's Vineyard Hospital for treatment and another ten refused treatment at the scene.

"There were a few serious injuries, none of them life-threatening, and a number of bruises and cuts," said Chilmark police chief Timothy Rich later in the evening.

Many of the cuts came from glassware that crashed off the deck along with the guests and also heavy clay flower pots that slid off the deck and broke into pieces.

Traffic up-Island was badly snarled for a couple of hours just after the incident. Cars were backed up from Beetlebung Corner past Squibnocket and there was gridlock at the Menemsha Pond boat launch. Some of the guests who had attended the party had parked their cars at the Quitsa overlook.

At 8 p.m. rescue workers packed the last victim, a blonde woman clutching an ice pack over a blackened eye, into a waiting ambulance. A few guests still lingered at the house, their cut hands wrapped in tape and bandages. The guests stared out at the water.

When the accident happened the moon was coming up on the first clear night the Vineyard had seen in two weeks. The planet Mars was rising brightly in the night sky. It was a day marked by oddities - earlier in the afternoon, a freak power outage had left all of New York city and many other sections of the Northeast without electricity.

The Chilmark accident sparked a flurry of press reports, including a banner on WCVB Channel Five television.

The cause of the accident was not immediately known; Chief Rich said last night that the accident is under investigation.

"The fire chief and building inspector will conduct a thorough investigation into the cause," the chief said.

A resident of Larchmont, N.Y., Ms. Lee is the widow of the late John J. Lee, a former All American basketball player and CEO of the Hexcell Corporation, who died two years ago.

When the chaos had calmed last night, Ms. Riger-Hull marveled that things had not been worse. "Considering what transpired, I'd say there was a whole pile of angels on the deck with those people," she said.

Gazette staff reporters Tom Dunlop, Jessie Royce Hill, Mandy Locke, Mark Alan Lovewell and Alexis Tonti contributed to this story.